Previously, we reported improved silicon epitaxy by hot-wire chemical vapor deposition (HWCVD) between about 600 and 650°C. Such temperatures are compatible with the thickening of large-grained Si seed layers on borosilicate glasses or other inexpensive substrates. Here, we provide detailed real-time spectroscopic ellipsometry (RTSE) and x-ray diffraction (XRD) analysis of two films grown near 600°C. A film grown at 594°C shows breakdown to a polycrystalline phase, while a film grown at 627°C is entirely epitaxial. Transmission electron microscopy (TEM) of this epitaxial film shows dislocation defects that originate at the substrate/film interface, suggesting that an optimized surface preparation could yield lower defect densities.

We grow epitaxial silicon films onto (100) silicon wafers from pure silane by hot-wire chemical vapor deposition (HWCVD). The films grow epitaxially for a thickness hepi before a Si:H cones nucleate and expand. We study the dependence of hepi on growth rate and the differences between Ta and W filaments. The surface morphology of thin but completely epitaxial films are studied in order to correlate the surface roughness during growth with the eventual epitaxial breakdown thickness. Surface roughness, strain and H at the wafer/film interface are not likely to cause the observed breakdown.

Solid state crystallization of hydrogenated amorphous silicon (a-Si:H) prepared by hot-wire CVD is studied in solid phase epitaxy mode. By using a novel optical method combined with cross-sectional TEM and SIMS, a reduction of epitaxial growth speed is observed with increase in a-Si:H film thickness. Namely, in films thinner that 0.5 micron, solid phase epitaxy velocity depends linearly on film thickness. As the film thickness increases beyond 1 micron, the average velocity of solid phase epitaxy decreases considerably with respect to that in thinner films. In this regime, its velocity becomes also time-dependent: initial slow propagation of crystallization front gets considerably accelerated after the front has traveled above 400nm. SIMS thickness profiles of hydrogen shows considerably more residual hydrogen in thicker films after the start of solid phase epitaxy. In addition, prolonged pre-dehydrogenation at lower temperatures results in the increase in the average epitaxy speed in thicker films. These phenomena are likely related to delayed hydrogen outdiffusion in thicker films, which also leads to time-dependent speed of the solid-phase epitaxy front. We conclude that excess residual hydrogen reduces the rate of solid-phase crystalline growth.

We measure times for complete solid phase crystallization (SPC) of hydrogenated amorphous silicon (a-Si:H) thin films that vary eight orders of magnitude, from a few ms to a few days. The time-to-crystallization activation energy is consistent with literature values of approximately 3.4 eV but the prefactor is markedly different for hot-wire chemical vapor deposition (HWCVD) films than for plasma-enhanced (PE) CVD films. The crystallized films were 0.3 – 2 μm thick, and deposited by high deposition rate (10-100 Å/s) HWCVD or standard PECVD onto glass substrates. We annealed these a-Si:H films over a wide temperature range (500 to 1100 °C) using techniques including simple hot-plates and tube furnaces, rapid thermal annealing by a tungsten-halogen lamp, and microwave electromagnetic heating at 2.45 GHz (magnetron) and 110 GHz (gyrotron).

We have studied silicon films grown epitaxially on silicon wafers using hot-wire chemical vapor deposition (HWCVD) with a tantalum filament. Silicon films were grown on (100)-oriented hydrogen terminated silicon wafers at temperatures from 175°C to 480°C, using a Ta filament 5 cm from the substrate to decompose pure SiH4 gas. The progression of epitaxy was monitored using real-time spectroscopic ellipsometry (RTSE). Analysis using RTSE, transmission electron microscopy (TEM), and scanning electron microscopy shows that at a characteristic thickness, hepi all of the films break down into a-Si:H cones. Below 380°C, both hepi and the thickness of the transition to pure a-Si:H increase with increasing temperature. Above 380°C, hepi was not observed to increase further but TEM images show fewer defects in the epitaxial regions. Secondary ion-mass spectrometry shows that the oxygen concentration remains nearly constant during growth (<1018 cm-3). The hydrogen concentration is found to increase substantially with film thickness from 5·1018 to 5·1019 cm-3, likely due to the incorporation of hydrogen into the a-Si:H cones that grow after the breakdown of epitaxy.

We use in-situ real-time spectroscopic ellipsometry to observe the breakdown of silicon epitaxy during growth by hot-wire chemical vapor deposition (HWCVD) on Si (100) substrates. Representative data is presented for the two types of epitaxy breakdown that we have observed: 1) an immediate transition to hydrogenated amorphous silicon (a-Si:H), and 2) a slower transition where a-Si:H cones nucleate and grow until they eclipse further epitaxial growth. Simple models, consistent with transmission-electron and atomic-force micrographs, describe the evolution of both types of breakdown showing that real-time spectroscopic ellipsometry is a useful tool for monitoring the growth of epitaxial silicon.

We incorporate narrow-gap amorphous silicon germanium (a-SiGe:H) alloys grown by hot-wire chemical vapor deposition (HWCVD) into single-junction n-i-p solar cells, and improve both fill factor (FF) and open-circuit voltage (Voc) by bandgap grading. The Tauc bandgap (ET) of the a-SiGe:H is as low as about 1.25 eV. Previously [1], we obtained a short-circuit current density (Jsc) up to 20 mA/cm2 in an n-i-p device incorporating an ungraded 120-nm i-layer of 1.25-eV a-SiGe:H. However, without buffer layers or bandgap profiling, the fill factor was only 38%, likely due to an abrupt bandgap transition and poor hole collection. To overcome these problems, we have used composition bandgap profiling throughout the i-layer and improved both Voc and FF significantly without any Jsc loss. The solar cell efficiency is improved from 3.55% to 5.85% and Voc rises from 0.475 to 0.550 eV. This improved single-junction a-SiGe:H solar cell has a quantum efficiency of about 48% at l=800 nm and about 15% at l=900 nm. We present details of the bandgap profiling and its effect on device performance.

We have achieved the best open-circuit voltage (Voc = 0.94 V) to-date in hydrogenated amorphous silicon (a-Si:H) photovoltaic cells deposited entirely by hot-wire chemical vapor deposition. The fill factor (FF = 0.74) remained high and a current density of 8-9 mA/cm2 with about 1800 Å i-layer was obtained in our n-i-p cells on untextured stainless-steel substrates. The Voc improvement of about 60 mV in compared to our previous best Voc was obtained by incorporating materials grown with H-dilution close to the phase transition from amorphous to microcrystalline silicon in the i-layer and at the i-p interface. A low substrate temperature of 150°C for the i-layer was also essential, most likely to widen the bandgap of the i-layer. A brief atomic H-treatment after grown the i-layer increases the Voc further by improving the p-i interface. The last 60 Å of the i-layer before p-layer is extremely close to the transition to microcrystallinity, though it remains mainly amorphous. Our p-layers are also close to the phase transition.

High quality amorphous silicon germanium (a-SiGe:H) alloys have been obtained using the hot wire chemical vapor deposition (HWCVD) from a gas mixture of SiH4, GeH4, and H2 at a deposition rate of ∼10 Å/s. Solar cells in a SS/n-i-p/ITO configuration are evaluated in which the n- and i-layers are deposited by HWCVD at NREL and the microcrystalline p-layer by conventional RF glow discharge in a separate reactor by United Solar. Effects of hydrogen dilution and step-wise bandgap profile have been studied and optimized. The best cell has an average optical bandgap of 1.6 eV and incorporates multi-bandgap steps where the narrow-most bandgap is near the p-i interface. J-V characteristics are measured under AM 1.5 illumination with a λ>530 nm filter. The best initial power output obtained exceeds 4 mW/cm2, which is usually used as an indicator for a good quality middle-gap cell. Double-junction cells are made on textured Ag/ZnO back reflectors. The bottom cell uses the optimized a-SiGe:H alloy cell by HWCVD, and the top cell uses an optimized a-Si:H cell near the amorphous-to-microcrystalline transition by PECVD at ∼1 Å/s. The best double-junction cell made to date exhibits an initial AM 1.5 active-area efficiency of 11.7%, and a stable efficiency after 1000 hours of one sun light soaking of 9.6%.

Efficient hydrogenated amorphous silicon (a-Si:H) n-i-p solar cells have been fabricated with all doped and undoped a-Si:H layers deposited by hot-wire chemical vapor deposition (HWCVD). The total deposition time of all layers, except the top ITO-contact, is less than 4 minutes. On an untextured stainless steel (SS) substrate, an initial efficiency of 7.12% is reached, with a stable efficiency of 5.4% after 1000 hours 1 sun light soaking. This initial efficiency is reached by incorporating into the p/i interface about 60 Å of intrinsic a-Si:H “edge” material grown under conditions near the transition to microcrystallinity. This edge layer increases the cell's fill factor from 0.60 to 0.68 and the best open-circuit voltage is about 0.88 V. Using textured Ag/ZnOcoated SS supplied by United Solar Corporation, preliminary results of an all-HWCVD solar cell give an initial efficiency of 8.7 %.

We have studied light-soaking effects, such as photoconductivity (PC) degradation kinetics, the changes of conductivity activation energy, Ea, and the defect density of states (DOS) in a-Si:H films deposited by hot-wire CVD. Films were deposited in a substrate temperature range from 280 to 440 °C for filament temperatures of 1900 and 2100 °C. We find that (a) the photodegradation kinetics does not follow the stretched exponential rule for all of the samples; (b) the Fermi level position moves up after light-soaking for most samples; and (c) the metastable defect DOS deduced from sub-band gap absorption is not consistent with that deduced from the electron mobility-lifetime product. The results are discussed according to the possible mechanism in which charged defects exist in hot-wire a-Si:H films.a

We grow hydrogenated amorphous silicon (a-Si:H) by Hot-Wire Chemical Vapor Deposition (HWCVD). Our early work with this technique has shown that we can grow a-Si:H that is different from typical a-Si:H materials. Specifically, we demonstrated the ability to grow a-Si:H of exceptional quality with very low hydrogen (H) contents (0.01 to 4 at. %). The deposition chambers in which this early work was done have two limitations: they hold only small-area substrates and they are incompatible with a load-lock. In our efforts to scale up to larger area chambers—that have load-lock compatibility—we encountered difficulty in growing high-quality films that also have a low H content. Substrate temperature has a direct effect on the H content of HWCVD grown a-Si:H. We found that making dramatic changes to the other deposition process parameters—at fixed substrate temperature and filament-to-substrate spacing—did not have much effect on the H content of the resulting films in our new chambers. However, these changes did have profound effects on film quality. We can grow high-quality a-Si:H in the new larger area chambers at 4 at. % H. For example, the lowest known stabilized defect density of a-Si:H is approximately 2 × 1016 cm-3, which we have grown in our new chamber at 18 Å/s. Making changes to our original chamber—making it more like our new reactor—did not increase the hydrogen content at a fixed substrate temperature and filament-to-substrate spacing. We continued to grow high quality films with low H content in spite of these changes. An interesting, and very useful, result of these experiments is that the orientation of the filament with respect to silane flow direction had no influence on film quality or the H content of the films. The condition of the filament is much more important to growing quality films than the geometry of the chamber due to tungsten-silicide formation on the filament.

We report progress in hydrogenated amorphous silicon n-i-p solar cells with the i-layer grown by the hot-wire chemical vapor deposition technique. Early research showed that we grew device-quality materials with low saturated defect density (2 × 106/cm3), high initial ambipolar diffusion length (~2000 Å) and low hydrogen content (<1%). One of the major barriers to implementing this material into solar cells is the high substrate temperature required (>400°C). We re-assess the effects of low substrate temperature on the property of the films and the performance of the solar cells as an alternative avenue to solving this problem. We find that the material grown at 300°C can have similar values of saturated defect density and ambipolar diffusion length as the one grown greater than 400°C. We also study the effect of i-layer substrate temperature ranging from 280° to 440°C for n-i-p solar cells. We now consistently grow devices with Fill Factor (FF) greater than 0.66, with the best close to 0.70 at lower substrate temperature. A collaboration with United Solar System, in where they grew the p-layer and top contact, produced devices with initial efficiencies as high as 9.8%. We produce n-i-p solar cells with initial efficiencies as high as 8% when we grow all the hydrogenated amorphous silicon and top contact layers. All these i-layers are grown at deposition rates of 16 to 18 Å/sec. We need to further improve our p-layer and transparent conductor layer to equal the collaborative cell efficiency. We also report light-soaking results of these devices.

We describe a series of microcrystalline (μc) silicon n-i-p solar cell devices fabricated entirely by the hot-wire chemical vapor deposition technique. These devices are deposited on flat stainless-steel at a substrate temperature below 250°C, and are evaluated using solar-cell performance and quantum-efficiency (QE) measurements. We explore the effect of crystallite size, as examined by X-ray diffraction, by varying the hydrogen-to-silane ratio from 5 to 40, while keeping the μc-n and the μc-p layers the same. We find a significant blue shift of the QE peak and an enhancement of red response compared with a standard a-Si:H solar cell. The blue shift increases with increasing hydrogen-to-silane ratio. We attribute this shift to the i-layer becoming more n-type with increasing hydrogen dilution. We also use a hydrogen gas purifier and find a large improvement in device performance.

Device-quality a-Si:H films were prepared by glow discharge CVD with pure or H-diluted silane as well as by hot-wire CVD. The hydrogen content was varied from ∼2 to 15 at. %. The Si-H bond absorption and its light-soaking-induced changes were studied by IR and differential IR absorption spectroscopes. The results indicate that the more stable sample exhibits an increase of the absorption at wave number ∼2000 cm−1, and the less stable one exhibits a decrease at ∼2040 cm−1and an increase at ∼1880 cm−1.

We observe that under certain conditions, hydrogenated amorphous silicon grown by the hotwire chemical-vapor deposition technique has dark conductivities varying by several orders of magnitude across the same film. Similarly, the ambipolar diffusion lengths fluctuate significantly in these films, yet there is not much evidence of a corresponding structural change. We attribute this electronic nonuniformity to electrons from the filament reaching insulating substrates and charging the substrate negatively in some regions, thus causing films to grow with inferior material properties in those regions. The effect diminishes with increasing substrate temperature, where the film itself may be conductive enough to remove charge reaching the growing surface. Wellgrounded, conducting substrates seem to be immune from this effect. We reduce this effect by putting a conductive grid on insulating substrates, of close enough spacing to remove the charge, and measure the material properties of the film grown between the conductive elements of the grid.

An innovative bending-beam method is used to study the stress of thin film a-Si:H deposited on thin quartz by hot-wire chemical vapor deposition (CVD) techniques. When the deposition temperature increases from 280 to 440 °C the hydrogen content decreases from 8 to <1 at.%, and the initial compressive stress also decreases from 420 to 74 MPa. We found that there is a 10−4 photo-induced increase of the initial compressive stress under 300 mW/cm2 light-soaking, which can be recovered to the initial value by thermal annealing at 160 °C for 1 h. The results imply that the Si-H bonds contribute to the compressive stress in the a-Si:H film. There is no simple correlation between the stress and the photodegradation of the electronic properties.

We present the results of studies on the defect properties and the effect of light soaking for various hot wire deposited (HW) films. We employ junction capacitance measurements together with the transient photocapacitance spectroscopy to measure the deep defect densities in as-grown state (state A) and in light soaked state (state B). Good agreement is found between the defect densities measured from both measurements. The HW film with a hydrogen content of 10 – 12 at.% shows physical characteristics and defect densities similar to conventional PECVD films. The HW films with hydrogen content, CH, in the range 2 – 9 at.% show a smaller defect density in state B than the defect density of the film with higher CH. However, the film with a hydrogen level of less than 1 at.% exhibits markedly inferior physical properties.

We observe reversible quenched-in metastability in the dark conductivity of extremely low-H-content amorphous silicon (a-Si). The sample is sputtered, P-implanted a-Si containing 0.1 at.% of H. Except for the high value of its equilibration temperature (T*), 355° ± 20°C, this metastability is similar to that observed in doped hydrogenated a-Si (a-Si:H). The ∼10 at.% H present in a-Si:H is not essential for a-Si metastability. After hydrogenation with about 10 at.% of H, T* falls to 175° ± 10°C. We propose that higher H content reduces T* by increasing Si network flexibility.